Compact optical tracking system



Jam: 3,3196? 5. c. ARGYLE 3,296,443

COMPACT OPTICAL TRACKING SYSTEM Filed Feb. 24, 1961 2 Sheets-Sheet l i 92 s 3 I g N T- m r a? :7 F :g I 5' ID ID a] II- r E N g m m INVENTOR.

SIDNEY C. ARGYLE ATTO NEY Jan? 1967 s. c. ARGYLE COMPACT OPTICAL TRACKING SYSTEM 2 Sheets-Sheet 2 Filed Feb. 24, 1961 HQVI'IOA "IVNSIS kLlSNI-LLNI .LHSll INVENTOR. SIDNEY C. ARGYLE ATTORNE 3,296,443 COMPACT OPTICAL TRACKING sYsTsM Sidney, .C. Argyle, Glendora, Calif., assignor to Aerojet- General Corporationp Azusa, Calif., a corporation of Ohio t t i Filed Feb. 24, 1961, Ser. No. 91,544

2 Claims. (Cl. 2Stl2ti3) This invention relates generally to a tracking device and in particular. to an optical infrared tracking head.

Infrared :tracking systemshave been used to direct vehicles such as guided missiles or aircraft to their tar- "gets; Suchtrackingsystems typically comprise an opticalt system combined with some kind of infrared detector. .They. operate in such a way that the output 1 signal from the detector varies when the object being i guided grnoves off target. in thevariation in the output signalis then fed into suitable servomechanismswhich steer the vehicle back to its The information contained proper course.

Pfiorinfrared tracking systems used to guide these vehicles were rather bulky and insensitive. This character- ,Areticleis positioned at the focal plane of the optical system. With this arrangement, the image of a distant objectpviewed by the optical system is sharply focused on the reticle, whilemoving thereon along a generally circularpath. If the object viewed is on the axis of the optical system, the generally circular image path will be; concentric with the optical axis. If the object viewed is off the optical axis, the generally circular image path will notbe concentric. with the optical axis.

.The reticle is provided with light transmitting porftions and opaque portions disposed in the path of the moving image. These light transmitting and opaque portions are so arranged that when the image path is concentric with the optical axis, the pulses of light passing through the reticlei will have a predetermined pattern.

When the object viewed is off the optical axis, the image "will pass over different parts of the light transmitting and opaqueportions of the reticle, producing thereby a different pattern of light pulses passing through the reticle.

Each position of an object off the optical axis produces a pattern of light pulses characteristic, of that position.

i The optical system also includes a field lens to focus the image of the entrance aperture of the optical system ontthe detector. With this arrangement, each light pulse passing through the reticle illuminates the image of the entrance aperture on the detector, causing it to produce a corresponding output signal.

Suitable analyzers and servomechanisms are connected togthevehicle being guided to alter its course in response 1 to the information contained in the detector output sig- 1 nal. In this way, the course of the vehicle may be altered to bring it back on target.

Other, objects of this invention will become more apf parent when. read in the light of the accompanying drawings, and specification, wherein:

FIG. 1 is a side view, partly in cutaway cross section, of the optical infrared tracking head.

FIG. 2 is a sectional view taken on the line 2-2 of FIG. 1, and showing in dotted lines the image point locus of on-targetobjects and the image point locus of off-target objects;

United States Patent 0 FIG. 3 shows the pattern of the intensity of light passing through the reticle and the signal output from the detector when the infrared tracking head is on target.

FIG. 4 shows one pattern of the intensity of light passing through the reticle and the signal output from the detector when the infrared tracking head is off target.

FIG. 5 is a sectional view taken on the line 5-5 of FIG. 1.

Referring now to FIG. 1 of the drawing, the infrared tracking head indicated generally by the reference numeral 10 comprises an optical system indicated generally by the reference numeral 12, and an infrared detector 14 mounted in optical contact in a'medium having a refractive index greater than that of air. It is understood that in optics it is common to refer to items in optical contact with one another as being immersed one with respect to another.

The elements of the optical system are mounted in a support 13 by any suitable means (not shown). As shown in the drawing, the optical system includes a Cassegrainian type telescope portion for viewing distant objects, an associated image nutator, and a field lens. The Cassegrainian portion comprises a spherical positive collector mirror 16 having a front surface R and a thick spherical negative mirror 18 having front surface R and a reflectice rear surface R Mirror 18 is positioned in front of the primary mirror 16 as shown in FIG. 1. In addition, a collimator lens 20 having front and rear surfaces R and R is mounted on the axis 24 of the optical system between mirrors 16 and 18. Lens 2ft cooperates with other optical elements to hold the image of the entrance aperture (in this case the collector mirror 16) at infinity, for reasons to become apparent below. The thick mirror 13 has the added function of controlling the major portion of the spherical abberations introduced into the optical system, but this correction introduces a considerable amount of chromatic abberation. The collimating lens 20 is therefore given the added function of controlling this chromatic abberation, but in doing so, this lens re-introduces a small amount of spherical aberration into the optical system. This spherical aberration, however, can be minimized by a judicious selection of the other optical components. If further control of the various abberations is desired, mirror 16 can be thick in order to introduce other corrections.

The image nutator 22 is mounted on the optical axis 24 of the optical system 12, and in this embodiment, the image nutator is coaxial with the Cassegrainian portion and generally concentric with mirror 16. This arrangement is highly desirable because it permits the optical system to be unusually light and compact without sacrificing its ability to view distant objects.

The image nutator comprises an inclined plate 26 mounted on the optical axis 24 adjacent rear surface R, on the collimating lens 20. Plate 26 has front and rear major planar surfaces R and R which are inclined to the optical axis 24 as shown in FIG. 1. The effect of the inclined plate 26 is to cause a generally radial shift of the image of an object viewed by the optical system. Thus, if the image of the object viewed by the optical system would fall on the optical axis 24 without the inclined plate, the addition of the inclined plate 26 would displace the image so that it falls off the optical axis. This displacement, however, introduces certain aberrations (coma and astigmatism) and to compensate for them, a spherical lens 32 having front and rear surfaces R and R is cemented to the front major planar surface R of the inclined plate 26. With this arrangement, the image of an object viewed by the optical system and on the optical axis would be sharply focused off the optical axis. Lens 32 has an added function in that it cooperates with the collimating lens 20 in holding the image of the entrance aperture or mirror 16 at infinity.

It is noted that there may be circumstances where severe control of chromatic aberration is not required. In such sinusoidally. This characteristic is desirable from the standpoint of frequency variation detection.

On the other hand, as seen in FIG. 2, when the object viewed by the optical system is off the optical axis, so

an event, the collim ating lens 20 may be omitted and sur- 5 that its image follows a circular path 50 which is not conface R of the inclined plate 26 may be ground so surcentric with the optical axis, the length of the opaque and faces R and R are no longer parallel to provide the light transmitting portions of a typical path 50 will no same end result as imaging the entrance aperture at inlonger be equal. Consequently, the intensity of light finity. passing through the reticle will no longer vary sinusoidally As seen in FIGS. 1 and 5, the inclined plate 26 and 10 at a constant frequency (see FIG. 4). It is also apparent spherical lens 32 are mounted in a tube 34 which is rotatathat the pattern of the light intensity passing through the bly supported by bearings 36. The tube is rotated by a reticle from objects not lying on the optical axis will vary motor 40 through a suitable drive. Thus, the image nutafrom point to point. tor causes the radially shifted image of an object viewed The behavior of the optical system so far described can by the optical system to move along a generally circular 15 be illustrated by following a bundle of rays represented by Path. line 52 coming from a point object off the optical axis 24. A reticle 42 is mounted on the optical axis 24 adjacent These rays pass through the hyper-hemispherical irdome the major planar surface R and in the focal plane of the 53 (in the case ofan infrared tracking head). The irdome optical system. Consequently, the sharply focused image is rigidly mounted on the vehicle and serves as a protective of an object viewed by the optical system moves along a cover for the optical components. After passing through generally circular path on the reticle. With this arrangeirdome 53, the rays fall on surface R of the spherical posiment, the image of an object viewed by the optical system tive collector mirror 16 and are then reflected to the and on the optical axis will move on the reticle along a spherical negative mirror 18 where they are refracted by generally circular path 48 (see FIG. 2), and this path is surface R reflected by surface R and again refracted by concentric with the axis of the optical system. On the surface R The rays then pass through the collimating other hand, if the object viewed by the optical system is ens 20 W re th y are refracted by surfaces R and R off the optical axis, its image will also move along some After refraction, the rays pass through the correcting lens generally circular path 50 (see FIG. 2). However, in 32 and inclined plate 26 where they are again refracted. this case, path 50 is no longer concentric with the optical on emerging hem Plate 25, the rays are focused on the axis. 30 reticle 42. As stated above, pulses of light pass through The reticle 42, in this particular embodiment, is divided the reticle, and this light then ImsseS through the field into aplurality of equal circular segments. This arrangelens assembly 55 t0 the detector AS ShOWn in the ment is desirable for reasons to become apparent below, drawings, the field lens assembly Comprises lenses but it is not essential, and segments having other shapes and are contemplated. Circular segments 44 are light trans- T function of the field lens m ly 55 is t imag mitting and circular segments 46 are opaque. As seen in the entrance aperture (in this case, mirror 16) on detector FIG. 2, the light transmitting segments 44 are in uniform 1 T detector as Stated above, may be Immersed angularly spaced relation to each other around the axis of in a medium having a refractive index greater h the optical system and are separated from each other by T is important because i makes the Size 0f the ge the opaque circular segments 46, as shown. Consequently, of the entrance aperture on the detector smaller. This the generally circular path 48 followed by the image of increases the flux density of t t Oh the e objects lying on the optical axis will have alternately that the System is m0feS enS1tiVe- Since the eolhmatlng spaced light transmitting and opaque portions of equal lens 20 in cooperation with spherical lens 32 causes the length. In addition, the size of the image 49 on the image of the entrance aperture to appear to be at infinity, reticle coming from an object lying on the optical axis, the image of the collector mirror 16 on the detector 14 is generally equal to the arc length of a circular segment will be stationary despite the movement of the target 44 along circular path 48. As a result, there is no abrupt image along the field path. transition as the image moves from the light transmitting 50 FIG. 1 shows a cross sectional view of the optical segments 44 to the opaque segments 46. Therefore, the system. The light is considered to travel from left to intensity of light passing through the reticle from the image right. The formula for the optical system described of objects lying on the optical axis will vary generally above is as follows:

Radii in Inches Thickness in Inches N1 in N1 1,. N2 5, v

1. 5028 1. 4988 1.4943 58.68 Air Air t 1.502s 1. 4988 1. 4943 58.68 1. 5028 1. 4938 1.4943 58.68 13:: 18th "rats 1. an an Air a 1 5028 it 15 1 8 50 l i 8458 i 8413 96. ii 7 an;

edit

its: 14

1 Optical cement. I I

Nita, represents refract ve ndex at 1.9 m crons.

NH, represents refractive ndex at 2.2 m crons.

N 2.5,. represents refractive index at 2.5 microns. t I v F represents the dispersion number for each element in the spectral region [ioiii 1.L/.1 to no With this arrangement, each time a pulse of light passes through one of the light transmitting portions 44 of the reticle, it will light up the image of the collector mirror 16 on :the detector causing the detector to emit a corresponding :pulse, Consequently, the output signal of the .detector will have the same general shape as the variation ofthe light intensity passing through the reticle.

This,means,;that for targets on the. axis of the optical I system; the output of the detector 14 will be a generally fixed frequency sine wave.

sponse to .the information contained in the frequency of the detector output signal. In this way, the course of t the vehicle is altered to bring the vehicle back on target.

It isunoted that the support 13. for the optical system and rear planar major surfaces each of which is inclined at an angle relative to said optical axis, a spherical correcting lens cemented on the front planar major surface of the transparent plate for correcting aberrations introduced by said plate, a reticle positioned on said optical axis adjacent the rear planar major surface of said plate and positioned so the image of an object in the field of view of the optical system is sharply focused thereon, means for rotating said transparent plate and said spherical correcting lens to cause the image on said reticle to move in a generally circular path, said reticle having at least one opening in part of the generally circular path of the image moving thereon, and a field lens mounted on the optical axis to the rear of said reticle to focus the image of the entrance aperture on the infrared detector so that any infrared light pulse passing through the reticle illuminates the image of the entrance aperture on the infrared detector and produces a corresponding output signal in the detector.

2. An optical system having numerical data substantially as follows:

Radii in Inches Thickness in Inches N13]: NM, NM :1

Air Air 1. 5028 1. 4988 1. 4943 58. 68 l. 1. 4988 1. 4943 58. 68

it 1. 8301 1. 8458 1. 8413 96. 14 1. 5028 1. 4988 l. 4943 58. 6

(l) (1) n 1. 8501 1. 8458 1. 8413 96. 14 1211.12, .187 Air 1112.13- 1. 8501 1. 8458 1. 8413 96. 14 12 4, All 1114.15, 1. 8501 1. 8458 1. 8413 96. 14 12 5. 11, All firms, 1. 8501 1. 8458 1. 8413 96. 14 l'hli, .00 (1) tisnn, 1. 5028 1. 4988 1. 4943 58. 68

1 Optical cement.

N represents refractive index at 1.9 microns.

N21,. represents refractive index at 2.2 microns.

N 2;. represents refractive index at 2.5 microns.

. represents the dispersion number for each element in the spectral region from 1.9;; to 2.5g.

is pivotally mounted by any suitable means (not shown) on pivot point70, at the centerof curvature of the irdome (see FIG. 1). This permits a large field of view to be searched without optical coupling or the introduction of objectionable aberrations. Although the irdome is shown I as generally spherical, it serves as an optical window.

Consequently, it could have other shapes, provided the shapes, do not introduce objectionable aberrations.

This invention is to be limited only in accordance with the scope of the appended claims, since others skilled in the art may devise other embodiments still within the limits of the, claims.

I claim:

LllAn infrared tracking head comprising in combination1,an-optical system and an infrared detector, said optical. system comprising a spherical positive mirror, a thick 'Isphericalnegative mirror positioned in front of the positive. mirror, 21 collimating lens positioned between the positive and negative mirrors to collimate the principalrays passing through the center of said optical system so thatthe principal rays. are made parallel to the axis of said optical system, a transparent plate mounted on the. axis OfSaid optical system, said plate having front References Cited by the Examiner UNITED STATES PATENTS JEWELL H. PEDERSEN, Primary Examiner.

BENJAMIN A. BORCHELT, ARTHUR M. HORTON,

Examiners.

P. G. BETHERS, S. W. ENGLE, E. S. BAUER,

Assistant Examiners. 

1. AN INFRARED TRACKING HEAD COMPRISING IN COMBINATION AN OPTICAL SYSTEM AND AN INFRARED DETECTOR, SAID OPTICAL SYSTEM COMPRISING A SPHERICAL POSITIVE MIRROR, A THICK SPHERICAL NEGATIVE MIRROR POSITIONED IN FRONT OF THE POSITIVE MIRROR, A COLLIMATING LENS POSITIONED BETWEEN THE POSITIVE AND NEGATIVE MIRRORS TO COLLIMATE THE PRINCIPAL RAYS PASSING THROUGH THE CENTER OF SAID OPTICAL SYSTEM SO THAT THE PRINCIPAL RAYS ARE MADE PARALLEL TO THE AXIS OF SAID OPTICAL SYSTEM, A TRANSPARENT PLATE MOUNTED ON THE AXIS OF SAID OPTICAL SYSTEM, SAID PLATE HAVING FRONT AND REAR PLANAR MAJOR SURFACES EACH OF WHICH IS INCLINED AT AN ANGLE RELATIVE TO SAID OPTICAL AXIS, A SPHERICAL CORRECTING LENS CEMENTED ON THE FRONT PLANAR MAJOR SURFACE OF THE TRANSPARENT PLATE FOR CORRECTING ABERRATIONS INTRODUCED BY SAID PLATE, A RETICLE POSITIONED ON SAID OPTICAL AXIS ADJACENT THE REAR PLANAR MAJOR SURFACE OF SAID PLATE AND POSITIONED SO THE IMAGE OF AN OBJECT IN THE FIELD OF VIEW OF THE OPTICAL SYSTEM IS SHARPLY FOCUSED THEREON, MEANS FOR ROTATING SAID TRANSPARENT PLATE AND SAID SPHERICAL CORRECTING LENS TO CAUSE THE IMAGE ON SAID RETICLE TO MOVE IN A GENERALLY CIRCULAR PATH, SAID RETICLE HAVING AT LEAST ONE OPENING IN PART OF THE GENERALLY CIRCULAR PATH OF THE IMAGE MOVING THEREON, AND A FIELD LENS MOUNTED ON THE OPTICAL AXIS TO THE REAR OF SAID RETICLE TO FOCUS THE IMAGE OF THE ENTRANCE APERTURE ON THE INFRARED DETECTOR SO THAT ANY INFRARED LIGHT PULSE PASSING THROUGH THE RETICLE ILLUMINATES THE IMAGE OF THE ENTRANCE APERTURE ON THE INFRARED DETECTOR AND PRODUCES A CORRESPONDING OUTPUT SIGNAL IN THE DETECTOR. 